Steel products and method for producing same



United States Patent 3,210,221 STEEL PRODUCTS AND METHOD FOR PRODUCING SAME Elliot S. Nachtman, Evanston, Ill., and John E. ODonnell, Highland, Ind., assignors to La Salle Steel Company, Hammond, Ind., a corporation of Delaware No Drawing. Filed May 29, 1961, 'Ser. No. 113,070 1 Claim. (Cl. 148-12) This application is a continuation-in-part of our application Serial No. 617,267, filed October 22, 1956, now Patent No. 3,009,843, for Steel Products and Method for Producing Same.

This invention relates to the improvement of mechanical and physical properties of steel in a process which is capable of use in the cold-finishing of steel, as in the cold-finishing of steel bars, rods, wires, tubing and the like steel products.

It is an object of this invention to produce and to provide a method for producing cold-finished steel products having new and improved physical properties, and it is a related object to produce a new and improved steel product having combinations of characteristics which differ from steels which have heretofore been produced.

More specifically, it is an object of this invention to provide a method applicable to the metallurgical processing of a hot-rolled steel, such as a steel of the type which strain-hardens and hardens by some mode of precipitation when worked at elevated temperatures to improve the physical and mechanical properties of the steel.

This application is an improvement over applications Serial Nos. 518,411; 518,412; 518,413 and 518,414, now Patents Nos. 2,767,837; 2,767,835; 2,767,836 and 2,767,838, which applications were copending with the parent application, Serial No. 617,267, now Patent No. 3,009,843 of which this application is a continuation-inpart.

In the aforementioned copending applications, the teaching is made of the new and improved concept for enhancing the physical and mechanical properties of steel by the combination of steps which includes the step of advancing the steel through a die to effect reduction in crosssectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition. This step is referred to hereinafter as the elevated-temperature-reduction step, which effected by working the steel to effect reduction int cross section while the steel is at a temperature within the range of 200 F. to 11001200 F. In accordance with the teaching of the aforementioned copending applications, the temperature employed in the elevated-temperature-reduction step influences the trend of impfovements secured in physical and mechanical properties of the steel. By controlling temperature, chemistry, and the amount of reduction, it becomes possible stiilectively to vary physical and mechanical properties eveloped in the steel. The properties can be varied ji iver a fairly wide range while enhancing and improving the uniformity of physical and mechanical characteristics developed in steels of the same chemistry from heat toIheat, as distinguished from the wide variations which -fiormally exist in hot-rolled steels.

It has now been found that the high-strength levels securedg by elevated-temperature-reduction of steels, as defined in the aforementioned copending applications, can be maintained along with many of the other improvements made available in the steels by the elevated-temperature-reduction step, and that still further improvements can be secured in the physical and mechanical properties of the steel to produce new and improved steel products when use is made of a combination of steps which includes heat-treating the steel after the elevated- 3,21,221 Patented Oct. 5, 1965 "ice temperature-reduction step, with or without a straightening operation in between.

As used herein, the term heat treatment after elevated-temperature reduction is meant to include:

1) Annealing the steel after the elevated-temperature reduction step by heating the steel to annealing temperature or to a temperature in the range of l400-1500 F. followed by slow cooling, as by means of air, to room temperature similarly to the annealing step employed in advance of elevated-temperature reduction.

(2) Normalizing the steel after the elevated-temperature-reduction step by heating the steel to normalizing temperature (above the upper critical temperature of the steel) or to a temperature between 1300 and 1500 F. followed by cooling in still air to room temperature.

(3) Producing a phase change in the steel after the elevated-temperature-reduction step by heating the steel to a temperature above the transformation rangethat is, to a temperature in the order of 15001600 F. and then quenching the steel to room temperature to produce a phase change to martensite.

(4) Producing a phase change in the steel after taking the elevated-temperature-reduction step, as by heating the steel to a temperature above the transformation range such as to a temperature within the range of 1500 to 1600 F., quenching the austenitized steel and then tempering the quenched steel by re-heating the steel to form tempered martensite.

(5) Processing the steel after the elevated-temperature-reduction step by heating the steel to a temperature above the transformation range, such as to a temperature within the range of 1500-1600 F., followed by a step quench to a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

(6) Marternpering the steel after taking the elevatedtemperature reduction, as by heating the steel to a temperature above the transformation range, that is, to a temperature above l500 F., and then quenching the steel in a bath maintained at a temperature in the upper portion of the temperature range for martensite forma tion (M until the metal has reached that temperature and then the steel is allowed to cool through the range for martensite formation to ambient temperature.

(7) Austempering the steel after taking the elevatedtemperature-reduction step by heating the steel to a temperature within the range of 1500l600 F. followed by quenching the steel in a bath maintained at a temperature slightly above the M range and of a character which causes the extraction of heat from the steel at a rate which prevents high temperature transformation products, and maintaining the steel at a temperature below that of pearlite formation but above that of martensite formation, such, for example, as when the steel is quenched in a salt bath maintained at a temperature of 650 F. followed by quenching to room temperature after the temperature of the steel has become uniform at the temperature of the quench.

(8) Treating the steel by diffusion transfer, as, for example, in the processes of carburizing, nitriding, cyaniding, chromizing, and carbonitriding the steel after the steel has been processed to the elevated-temperature-reduction step. As used herein, the terms diffusion and carburizing are intended to have their normal meaning in the steel trade. Diffusion can be defined briefly as involving the movement of atoms within a solution. The net movement is usually in the direction from regions of high concentration towards regions of lower concentration in the attempt to achieve homogeneity of the system, which may be in a liquid or gas or, as in the present instance, a solid metal. Carburizing is a diffusion transfer process wherein carbon is introduced into the steel by bringing the steel into intimate contacting relation with a'carbonaceous material, in the form of a solid, liquid or gas, and heating to a temperature above the transformation range for the steel and holding at that temperature for an amount of time sufficient to give the desired case to the steel. Carburizing is generally followed by quenching to produce a hardened case. The time and temperature relationship influences the depth of the case, as is well known in the art.

(9) Stress relieving without a phase change by heating the steel after elevated temperature reduction to a ternperature above 300 F. but below 1200 F., followed by rapid or slow cooling to ambient temperature.

It is believed that the elevated-temperature-reduction step permits taking of the desired reduction while the steel is in an easy-to-draw or soft condition, and that the elevated-temperature reduction affects the austenitic grain size, the transformation temperature range and the lower critical temperature range, and that it also has some effect on the adjustment of the subsequent austenitizing temperature and the M temperature range of the processed steel. Thus, the characteristics of the steel are changed by elevated-temperature reduction, such that subsequent heat treatment by the techniques described tends, with or Without a straightening step in between, to produce a new and different steel product having new and difierent properties and characteristics and combinations thereof by comparison with steels of the type which have heretofore been produced by the various conventional techniques and processes described in the aforementioned copending applications.

As used herein, the term physical and mechanical properties is meant to include tensile strength, yield strength, machinability, hardness, ductility, elongation, residual stress, warpage factor, and the like. It has been found, for example, as described in the aforementioned copending applications, that machinability characteristics of the steel and that such properties as tensile strength, yield strength, proportional limit, impact strength, and hardness are beneficially affected by reduction at elevated temperature, and that these properties are maximized, as described in the copending applications Ser. No. 518,414, now Patent No. 2,767,838 and Ser. No. 518,413, now Patent No. 2,767,836, when the elevated-temperaturereduction step is taken while the steel is at a temperature within the range of 450850 F. Even within this particular temperature range, it has been found that tensile strength, yield strength, and proportional limit will be maximized when the elevated-temperature-reduction step is taken while the steel is at a temperature within the range of 450600 F., while the more noticeable improvements in the plastic properties of the steel, as represented by elongation, reduction of area, and impact strength, will be maximized when the elevated-temperature-reduction step is taken while the steel is at a temperature within the range of 600850 F. These same concepts tend to carry over into the process of this invention, wherein the described elevated-temperature-reduction step is employed in advance of one of the described heat treatments, whereby still further modification and improvements in the characteristics and properties of the steel are secured.

Aside from and in addition to the described improvements in physical and mechanical properties, the elevatedtemperature-reduction step provides means by which a desirable control can be provided over the stress characteristics that are developed in the steel products. When, for example, the steel is advanced through a die to effect reduction in cross-sectional area While the steel is at a temperature in excess of 650 F., and preferably at a temperature above 850 F. but below the lower critical temperature for the steel composition, the magnitude of the residual stresses developed in the steel can be materially reduced and the type of residual stresses can be controlled to produce steel products having greatly improved warpage characteristics and a much more desirable distribution of stresses through the cross section of the steel.

The marked reduction in warpage value secured when the steel is reduced while at elevated temperature permits the production of steel products having improved physical and mechanical properties and residual-stress values as low as or lower than those which have been heretofore capable of being developed by the use of stress-relieving steps following drawings or an equivalent reduction process. It is possible to produce steel products having compressive stresses, as distinguished from tensile strengths as decribed in the aforementioned copending application Ser. No. 518,412, now Patent No. 2,767,835, when the steel is advanced through a die to eifect reduction in crosssectional area while the steel is at a temperature above 800 F. but below the lower critical temperature, and when the steel is rapidly cooled, as by quenching, immediately after the elevated-temperature-reduction step.

As in the aforementioned copending applications, the trend of the improvements in physical and mechanical properties and the extent of improvement in any one of the properties is influenced to some degree by the temperature of the steel being reduced, the chemistry of the steel, and the amount of reduction that is taken and the type of heat treatment which is employed following the elevated temperature reduction step.

The concepts of this invention can be practiced with steels classified quite generally as steels of the type which are characterized by the ability to strain harden and harden by some mode of precipitation when worked at an elevated temperature. Typical of the class of steels are the non-austenitic steels having a pearlitic structure in a matrix of free ferrite, and steels which are generally referred to as the easy to draw steels. The steels proc essed in accordance with the practice of this invention may be in the form of bars, rods, wire, tubing, or the like.

For effecting reduction at elevated temperature, the steels may be advanced through a die such as a draw die, extrusion die, or roller die, to effect reduction in cross-sectional area. While the process of rolling is not equivalent to the process of drawing or extrusion in a cold-finishing operation, it has been found that many of the characteristics and properties capable of being developed by drawing or extrusion at elevated temperature are also evidenced in steels which are processed in a rolling operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

The concepts of this invention will be illustrated by two different process cycles. The first will be elevated temperature followed by austenitizing and quenching. Reference to two steels which may be taken as representative of the steels that can be employed in the practice of this invention will be made. These representative steels will hereinafter be referred to as C-1144 and 4140 having the following ladle analysis, in which the major ingredients, other than iron, are set forth:

The hot-rolled steel bars, as received, were de-scaled by pickling in sulphuric acid and limed to prevent rusting. The hot-rolled, pickled, and limed bar stock was heated to an elevated temperature for advancement through a draw die to elfect reduction in cross-sectional area while the steel was at a temperature within the range of 200 F.

to the lower critical temperature for the steel composition. In the illustration of the practice of this invention, the elevated-temperature-reduction step was carried out while the steel was at two representative temperatures within the range described. The steel was heated to the desired temperature range for the elevated-temperaturereduction step by a conventional or other suitable heating furnace.

The steel, at the desired elevated temperature, was then lubricated with a conventional drawing compound and advanced through the draw die to take a representative reduction in cross-sectional area. The 1144 steel, originally of round, was given a 21.6% reduction, while the 4140 steel, of round, was given a 19.9% reduction.

After the elevated-temperature-reduction step, the steels were heated to austenitizing temperature for one hour. The temperature employed was 1500 F., and then the austenitized steels were quenched in oil to room temperature.

Table I It is to be understood that other temperatures than those illustrated in Tables III and IV may be used in the teachings of this invention and although not the same, rolling of the steel may be carried out at the elevated temperature to produce similar results.

It will be apparent from the foregoing examples that steels having new and different properties are secured by the processing of the steels to take an elevated temperature reduction step followed by heat treating, as by austenitizing and quenching, stress relieving, etc. Steels having reduced stress relationships and improved impact strengths are secured while maintaining the strength levels of the steel at a high level. Elevated temperature reduction can be followed by straightening by processing the steel bars or rods between straightening rolls whereby other stresses and characteristics are introduced into the steel before the heat treating step.

It will be understood that changes may be made in the details with respect to the methods of heating and cooling the steel, or in the techniques of processing the [0-1144-drawn to take a 21.6% reductionaustenitized at 1500 F. for one hourquenched to room temperature] [4140drawn to take a 19.9% reduction-austenitized at 1500 F. for one hour-quenched to room temperature] Austenitiz- Tensile Izod im- Hardness, Re Temp. of draw, ing temp., strength, pact, 70 Warpage F. F. p.s.i. F., ft.-lbs. S MR 0 Also included among the processes of heat treating after elevated temperature drawing is the process of stress relieving after elevated temperature drawing. This cycle representative of the teachings of this invention will be illustrated using C-ll44 steel as before.

Thc Bteel after pickling and liming is drawn at elevated temperatures, allowed to cool to room temperature and subsequently reheated to the stress relieving temperature. TableseI-II and IV will illustrate results obtained using such agleycle with two different percent reductions in area.

an Table III e [0-1144 steel drawn to take a 20.3 percent reduction] Tem H Stress Tensile Yield Elonga- Bed. of draw, F. relief temp., strength, strength, tion, area,

F. p.s.i. p.s.i. percent percent .5; Table IV [0-1144 steel drawn to take a 35.6 percent reduction] Temp. of Stress Tensile Yield Elonga- Bed. of draw, F. relief temp., strength, strength, tion, area,

F. p.s.i. p.s.i. percent percent steels through the various steps embodying the practice of this invention without departing from the spirit of this invention, especially as defined in the following claim.

We claim:

The metallurgical process for treating a steel which strain hardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition consisting of the following combination of steps in the order specified of plastically deforming the steel to effect a reduction in cross-sectional area while the steel is at a temperature within the range of 450 F. to the lower critical temperature for the steel composition, straightening the steel, heating the steel to a temperature above 300 F. but below 1200 F. for strain relieving, and then cooling the steel to about ambient temperature.

References Cited by the Examiner UNITED STATES PATENTS 1,957,427 5/34 Buchholtz 148--12 2,789,069 4/57 Nachtman 148-4 2,859,143 11/58 Nachtman et al. 148-2 3,009,843 11/ 61 Nachtman et al. 148-12.1

FOREIGN PATENTS 294,246 10/29 Great Britain. 761,151 11/56 Great Britain.

(Other references on following page) 3,210,221 'i 8 OTHER REFERENCES The Making, Shaping and Treating bf Steel, 7th ed., The Making, Shaping and Treating of Steel, 6th ed., Copyright 1957' pg. 1242. Copyright 1951. Pub. by US Steel (30., D AVID L. RECK Primary Examiner- I Pittsburgh, Pa.

The Making, Shaping and Treating of Steel, 7th ed., 5 RAY K. WINDHAM, ROGER L. CAMPBELL, pg. 815. Copyright 1957. Pub. by US. Steel Corp, WINSTON A. DOUGLAS, Examiners.

Pittsburgh, Pa. 

